Nitride semiconductor laser and method of fabricating the same

ABSTRACT

A method for fabricating a nitride semiconductor laser device having crystal layers each made of a group III nitride semiconductor (Al x Ga 1−x ) 1−y In y N (0≦x≦1, 0≦y≦1) layered in order on a ground layer (Al x′ Ga 1−x′ ) 1−y′ In y′ N (0≦x′≦1, 0≦y′≦1). The method including a step of forming a plurality of crystal layers each made of group III nitride semiconductor on a ground layer formed on a substrate such as sapphire; a step of applying a light beam from the substrate side toward the interface between the substrate and the ground layer thereby forming the decomposed-matter area of a nitride semiconductor; a step of separating the ground layer carrying the crystal layers from the substrate along the decomposed-matter area; and a step of cleaving the ground layer thereby forming a cleavage plane of the crystal layers.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a group III nitridesemiconductor device (hereafter also referred to as a device simply)and, particularly to a fabrication method of a semiconductor laserdevice using the same material system.

[0003] 2. Description of the Related Art

[0004] A laser device needs to have a resonator consisting of a pair offlat parallel mirror facets for its operation. For example, in the caseof the manufacture of a conventional laser device (Fabry-Perot type)using a semiconductor crystal material such as GaAs, the cleavage natureof GaAs crystal i.e., the substrate crystal is utilized for thefabrication of the mirror facets.

[0005] In the case of a group III nitride semiconductor device, it isinevitable to perform the epitaxial-growth of the crystal film onto adissimilar substrate such as sapphire, SiC or the like, because anitride bulk crystal is extremely expensive to be used in practicealthough it could be manufactured.

[0006] SiC is not frequently used as a substrate for the nitridedevices, because SiC substrates are also expensive and a nitride film onthe SiC substrate easily cracks due to the difference in thermalexpansion coefficient therebetween. Thus sapphire is commonly used as asubstrate for the group III nitride semiconductor laser devices. In thecase of epitaxial growth of nitrides on a sapphire substrate, a highquality single-crystal film is obtained on a C-face i.e., (0001) planeof sapphire, or on an A-face, i.e., (11{overscore (2)}0) plane(hereafter referred to as (11-20) plane) of sapphire.

[0007] The mirror facets may be formed by an etching process such asreactive ion etching (RIE) instead of cleavage, because it is hard tosplit the sapphire substrate to laser bars in comparison with the GaAssubstrate having been used so far for semiconductor laser devices.

[0008] Reactive ion etching is mainly used as a method for obtaining themirror facets of the nitride semiconductor laser on the sapphiresubstrate at present. However, the resultant device with the mirrorfacets formed by the reactive ion etching method has a disadvantage thatthe far-field pattern of its emitted light exhibits multiple spots. Themass-production-type GaN laser with the cleaved mirror facets is studiedagain in view of overcoming the multiple spots phenomenon in the farfield pattern as mentioned above.

[0009] It is a matter of course that the cleavage cannot be preferablyperformed on sapphire in mass production. Therefore, the followingmethod have been used. First, after forming a thick ground layer of GaNfilm e.g., at approximately 200 μm thickness on a sapphire substrate,the backside of the sapphire substrate of the obtained wafer is groundor lapped to remove the sapphire portion, so that the GaN substrate isobtained. Next, the epitaxial growth of laser structure is preformed onthe GaN substrate. From the obtained wafer, laser devices may befabricated.

[0010] However, the conventional method of lapping the back side thesapphire substrate backside as described above requires many steps, andis complicated. As a result, the method invites a very low yield of thegroup III nitride semiconductor devices. Such a method is not suitablefor mass production.

[0011] Although sapphire does not have a definite cleavage plane like aSi or GaAs wafer, a C-face sapphire is fairly easily split along its1{overscore (1)}00) plane (hereafter referred to as (1-100) plane), andalso an A-face sapphire can be easily parted along its (1{overscore(1)}02) plane (hereafter referred to as (1-102) plane), so calledR-plane, considerably close to the cleavage of ordinary crystal. It isconsidered that the formation of the mirror facets of nitridesemiconductor lasers on a sapphire substrate may be achieved throughfollowing methods: First is a method of growing nitride semiconductorlayers on a C-face sapphire substrate and then splitting the wafer along(1-100) plane of the sapphire substrate. Second is a method of growingnitride semiconductor layers on an A-face sapphire substrate and thensplitting the wafer along (1-102) plane of the sapphire substrate.

[0012] As to the first method of mirror facet formation applied to thedevice grown on a C-face sapphire substrate, there are problems that asapphire substrate cannot be split unless the substrate is made thinenough by lapping down the backside of the substrate and that it doesnot have high reproducibility. These problems are caused by the factthat (1-100) plane of sapphire is not an explicit cleavage plane. Sincesapphire is very hard crystal, it cannot be split exactly along a linenotched on its surface unless it is made thin enough, and the thicknessof the sapphire substrate should be reduced to approximately 100 μm inorder to obtain mirror facets practical for laser devices. When lappingthe backside of a wafer on which a device structure is already formed,the wafer is warped or distorted due to the difference between thermalexpansion coefficients of sapphire and nitrides or due to the residualstress caused by lapping process. When the back of a device wafer islapped, the wafer is thereby apt to fracture during the process. This isvery disadvantageous for mass production. The (1-100) plane of sapphireis not a cleavage plane. Therefore, in many cases GaN is split along ina direction slightly deviated from the cleavage plane thereof, thefracture surface consists of many facts of (1-100) planes of GaN, eachof which is the cleavage plane, forming a stepwise appearance. Thestepwise appearance causes degradation of the reflectivity andperturbation of the wave front of emitted light and, therebydeteriorates the quality of mirror facets for optical resonance of alaser device.

[0013] Whereas, the second method of mirror facet forming method appliedto the device formed on an A-face sapphire substrate has a problem thatthe quality of the fracture plane of GaN is not sufficient.

[0014] Since the sapphire substrate can be easily split along itscleavage plane (1-102), so called R-plane, it is possible to cleave thesapphire having a thickness of 250 to 350 μm normally used as asubstrate. However, as shown in FIG. 1, when forming a laser structureon the A-face of a sapphire substrate and parting sapphire along itsR-plane as depicted by the arrow in the figure, fine striations areformed on the side surface of GaN layers. This is caused by thefollowing reason that the laser wafer splits along the R-plane of thesapphire since a major part of the wafer is made of sapphire. TheR-plane of sapphire tilts by an angle of 2.4° from (1-100) plane of thegrown GaN as shown in FIG. 2, after a propagating crack along thesapphire's R-plane reaches at the sapphire-GaN interface, the crackstill propagates into GaN still along the R-plane of sapphire up to acertain depth. However, GaN tends to crack on its crystallographiccleavage plane (1-100). Therefore, a plurality of (1-100) facets of GaNare formed in such a stepwise manner that the striations appear on thefracture plane of GaN as shown in FIG. 1.

[0015] As a result, in the case of the A-face sapphire substrate, thequality of fracture plane is not very good though it is reproducible.

OBJECT AND SUMMARY OF THE INVENTION

[0016] Therefore, an object of the present invention is to provide agroup III nitride-semiconductor laser having high-quality mirror facetsfor a laser structure and a method of fabricating the laser device withhigh reproducibility.

[0017] A fabrication method according to the present invention is amethod for producing a nitride semiconductor laser device having crystallayers each made of a group III nitride semiconductor(Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1, 0≦y≦1), layered in order on aground layer (Al_(x′)Ga_(1-x′))_(1-y′)In_(y′)N (0≦x′≦1, 0≦y′≦1), themethod comprising the steps of:

[0018] forming a plurality of crystal layers each made of group IIInitride semiconductor on a ground layer formed on a substrate, thecrystal layers including an active layer;

[0019] applying a light beam from the substrate side toward theinterface between the substrate and the ground layer thereby forming thedecomposed-matter area of a nitride semiconductor;

[0020] separating the ground layer with the crystal layers thereon fromthe substrate along the decomposed-matter area; and

[0021] cleaving the ground layer thereby forming a cleavage plane of thecrystal layers for a laser resonator.

[0022] In an aspect of the fabrication method according to theinvention, the wavelength of said light beam is selected fromwavelengths passing through the substrate and absorbed by the groundlayer in the vicinity of the interface.

[0023] In another aspect of the fabrication method according to theinvention, the method further comprises, between said step of formingthe crystal layers and said step of applying the light beam toward theinterface, a step of bonding a cleavable second substrate onto a surfaceof the crystal layers in such a manner that a cleavage plane of thesecond substrate substantially coincides with a cleavage plane of thecrystal layers of the nitride semiconductor.

[0024] As to a further aspect of the fabrication method according to theinvention, in the step of applying the light beam toward the interface,the light beam is applied uniformly or entirely onto the interfacebetween the substrate and the ground layer.

[0025] As to a still further aspect of the fabrication method accordingto the invention, in the step of applying the light beam toward theinterface, the interface between the substrate and the ground layer isscanned with a spot or line of the light beam.

[0026] In another aspect of the fabrication method according to theinvention, the method further comprises a step of forming a waveguideextending along a direction normal to the cleavage plane of the nitridesemiconductor.

[0027] In a further aspect of the fabrication method according to theinvention, the crystal layers of the nitride semiconductor are formed bymetal-organic chemical vapor deposition.

[0028] As to a still further aspect of the fabrication method accordingto the invention, in the step of applying the light beam toward theinterface, the light beam is an ultraviolet ray generated from afrequency quadrupled YAG laser.

[0029] In addition, a nitride semiconductor laser device according tothe present invention having successively grown crystal layers each madeof a group III nitride semiconductor (Al_(x)Ga_(1−x))_(1−y)In_(y)N(0≦x≦1, 0≦y≦1) comprises:

[0030] a ground layer made of group III nitride semiconductor(Al_(x′)Ga_(1−x′))_(1−y′)In_(y′)N (0≦x′≦1, 0≦y′≦1);

[0031] a plurality of crystal layers each made of group III nitridesemiconductor formed on the ground layer;

[0032] a cleavable substrate bonded onto a surface of the crystal layersopposite to the ground layer.

[0033] In another aspect of the nitride semiconductor laser deviceaccording to the invention, the device further comprises a heat sinkbonded onto the ground layer.

[0034] In a further aspect of the nitride semiconductor laser deviceaccording to the invention, the device further comprises a heat sinkbonded onto the cleavable substrate.

[0035] In a still further aspect of the nitride semiconductor laserdevice according to the invention, the cleavable substrate has acleavage plane coinciding with a cleavage plane of the crystal layers ofthe nitride semiconductor.

[0036] In another aspect of the nitride semiconductor laser deviceaccording to the invention, the device further comprises a waveguideextending along a direction normal to the cleavage plane of the nitridesemiconductor.

[0037] In a further aspect of the nitride semiconductor laser deviceaccording to the invention, the cleavable substrate is made ofsemiconductor single-crystal such as GaAs.

[0038] In a still further aspect of the nitride semiconductor laserdevice according to the invention, the cleavable substrate is made of anelectrically conductive material.

[0039] According to the present invention, it is possible to obtainhigh-quality mirror facets by untying the crystal bond between thesapphire substrate and the ground layer of GaN crystal and separatingthe substrate and the ground layer and thereby, fabricating the laserdevice with high reproducibility.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a schematic perspective view showing the fractured planeof a GaN crystal layer formed on a sapphire substrate;

[0041]FIG. 2 is a schematic perspective view showing the lattice planeof a GaN crystal layer formed on a sapphire substrate;

[0042]FIG. 3 is a schematic sectional view of a group IIInitride-semiconductor laser device of an embodiment according to thepresent invention;

[0043]FIG. 4 is an enlarged schematic sectional view of a group IIInitride-semiconductor laser device of an embodiment according to thepresent invention, which is seen from the mirror facet for opticalresonance;

[0044]FIGS. 5 and 6 are schematic sectional views each showing a portionof a wafer for the semiconductor laser device at each fabricating stepof an embodiment of the present invention;

[0045] FIGS. 7 to 9 are schematic perspective views each showing aportion of a wafer for the semiconductor laser device at eachfabricating step of an embodiment of the present invention;

[0046]FIG. 10 is an enlarged schematic sectional view showing a wafer inthe semiconductor-laser fabricating step of an embodiment of the presentinvention;

[0047] FIGS. 11 to 16 are schematic perspective views each showing awafer in the semiconductor-laser fabricating step of an embodiment ofthe present invention;

[0048]FIG. 17 is a schematic sectional view of a group IIInitride-semiconductor laser device of another embodiment according tothe present invention;

[0049]FIG. 18 is an enlarged schematic sectional view of a group IIInitride-semiconductor laser device of another embodiment according tothe present invention, which is seen from the mirror facet for opticalresonance;

[0050]FIG. 19 is a schematic sectional view showing a portion of a GaAssubstrate for the semiconductor laser device at each fabricating step ofanother embodiment of the present invention;

[0051] FIGS. 20 to 23 are schematic perspective views each showing awafer in the semiconductor-laser fabricating step of another embodimentof the present invention; and

[0052]FIGS. 24 and 25 are enlarged schematic sectional views eachshowing a wafer in the semiconductor-laser fabricating step of anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] Embodiments of group III nitride semiconductor laser devicesaccording to the present invention are described below by referring tothe accompanying drawings.

[0054]FIG. 3 generally shows an embodiment of the group III nitridesemiconductor laser device of a refractive index guided type accordingto the invention. This device is constructed with a laser body 100, asupport substrate 200 bonded onto the laser body 100, and a chip carrier10 bonded onto the laser body 100 serving as a heat sink. The chipcarrier 10 is made of an electrically conductive material. The laserbody 100 comprises a ground layer 103 made of group III nitridesemiconductor (Al_(x′)Ga_(1−x′))_(1−y′)In_(y′)N (0≦x′≦1, 0≦y′≦1),crystal layers 104 to 110 each made of a group III nitride semiconductor(Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1, 0≦y≦1) successively grown in orderon the ground layer, and an electrode layer 113. The crystal layersinclude an active layer. The support substrate 200 is a cleavable orpartable substrate made of an electrically conductive material orpreferably semiconductor single-crystal such as GaAs, InP, Si, or thelike. The support substrate 200 is bonded to a surface of the crystallayers opposite to the ground layer 103 via the electrode layer 113. Acleavage plane of the support substrate 200 coincides with a cleavageplane of the crystal layers of the nitride semiconductor. Namely, thesupport substrate 200 is bonded to the surface of the crystal layers insuch a manner that the cleavage plane of the second substratesubstantially coincides with a cleavage plane of the crystal layers ofthe nitride semiconductor. The surface of the ground layer 103 of thelaser body 100 is bonded onto the chip carrier 10 via which the deviceis electrically connected to an external electrode. The laser body 100has a ridge waveguide extending along a direction normal to the cleavageplane of the nitride semiconductor layers 103 to 110 (which is thedirection normal to the drawing).

[0055] As shown in FIG. 4, the laser body 100 of the semiconductor laserdevice is constituted of the ground layer 103 i.e., n-type GaN layer103, an n-type Al_(0.1)Ga_(0.9)N layer 104, an n-type GaN layer 105, anactive layer 106 mainly containing InGaN, a p-type Al_(0.2)Ga_(0.8)Nlayer 107, a p-type GaN layer 108, a p-type Al_(0.1)Ga_(0.9)N layer 109,and a p-type GaN contact layer 110 which are layered on the ground layer103 in this order. A ridge stripe portion 118 is formed in the p-typeAl_(0.1)Ga_(0.9)N layer 109 and the p-type GaN contact layer 110 so asto extend along a direction normal to the cleavage plane of the nitridesemiconductor layers. The top of the laser body 100 is covered with andprotected by an insulating film 111 made of SiO₂ except a contact windowto the p-type GaN contact layer 110 of the ridge stripe portion 118. Theinsulating film 111 is covered with the p-side electrode layer 113. Then-type GaN ground layer 103 is connected to the chip carrier 10. Thep-side electrode 113 connected through a slit of the insulating film 111to the p-type GaN contact layer 110 is connected to the supportsubstrate 200 via a metal film.

[0056] The semiconductor laser device emits light by recombiningelectrons with holes in the active layer 106. The n-type GaN layer 105and p-type GaN layer 108 serve as guiding layers. Light generated in theactive layer 106 is guided in the guiding layers 105 and 108. Electronsand holes are effectively confined into the active layer 106 by settingband gaps of the guiding layers 105 and 108 to values larger than thatof the active layer 106. The p-type Al_(0.2)Ga_(0.8)N layer 107 servesas a barrier layer for further enhancing the confinement of carriers(particularly, electrons), the n-type Al_(0.1)Ga_(0.9)N layer 104 andthe p-type Al_(0.1)Ga_(0.9)N layer 109 serve as cladding layersrespectively each formed to have refractive indexes lower than those ofthe guiding layers 105 and 108. The wave-guiding in the lateraldirection is performed by the difference between refractive indexes ofthe cladding layer and the guiding layer. The ridge stripe portion 118is formed in order to produce a lateral-directional step in effectiverefractive index by changing the thickness of the cladding layer 109,thereby confining the generated light in the lateral direction.

[0057] The device structure shown in FIGS. 3 and 4 is fabricated in thefollowing fabricating steps in which layered structure for a laserdevice is formed through the metal-organic chemical vapor deposition(MOCVD) on an A-face sapphire substrate whose both sides aremirror-finished.

[0058] <Preparation of a Laser Wafer>

[0059]FIG. 5 shows a sectional view of a target laser wafer preparedthrough the following steps in which crystal layers for a GaNsemiconductor laser structure are grown on a sapphire substrate.

[0060] First, a sapphire substrate 101 is set into an MOCVD reactor andheld for 10 minutes in a hydrogen-gas flow at a pressure of 300 Torr anda temperature of 1050° C. to thermally clean the surface of the sapphiresubstrate 101. Thereafter, the temperature of the sapphire substrate 101is lowered to 600° C., and ammonia (NH₃) which is a nitrogen precursorand TMA (trimethyl aluminium) which is an Al precursor are introducedinto the reactor to deposit a buffer layer 102 made of AlN up to athickness of 20 nm. The GaN (or AlN) layer 102 formed at a lowtemperature acts as a buffer layer to ensure a growth of GaN film on thesapphire substrate which is a dissimilar material to GaN.

[0061] Subsequently, the supply of TMA is stopped, the temperature ofthe sapphire substrate 101 on which the buffer layer 102 is formed israised to 1050° C. again while flowing only NH₃, and trimethyl galliumis introduced to form the n-type GaN ground layer 103 on the bufferlayer 102. In this case, Me-SiH₃ (methyl silane) is added into a growthatmosphere gas as the precursor of Si which serves as an n-typeimpurity.

[0062] When the n-type GaN ground layer 103 is grown up to approximately4 μm, TMA is introduced to form the n-type AlGaN cladding layer 104.When the n-type AlGaN cladding layer 104 is grown up to approximately0.5 μm, the supply of TMA is stopped to grow the n-type GaN guidinglayer 105 up to 0.1 μm. When growth of the n-type GaN guiding layer 105is completed, the supply of TMG and Me-SiH₃ is stopped, and lowering oftemperature is started to set the substrate temperature at 750° C.

[0063] When the substrate temperature reaches to 750° C., carrier gas isswitched from hydrogen to nitrogen. When the gas-flow state isstabilized, TMG, TMI, and Me-SiH₃ are introduced for growing a barrierlayer in the active layer 106. Subsequently, the supply of Me-SiH₃ isstopped and then the flow rate of TMI is increased so that a well layerhaving an In composition ratio greater than that of the barrier layer isgrown on the barrier layer. The growths of the barrier layer and thewell layer are repeated in pairs in accordance with the number of wellsin the designed multiple quantum well structure. In this way, the activelayer 106 of the multiple quantum well structure is formed.

[0064] When the growth of active layer is finished, the supply of TMG,TMI, and Me-SiH₃ is stopped, and the carrier gas is switched fromnitrogen to hydrogen. When the gas-flow is stabilized, the substratetemperature is raised to 1050° C. again and TMG, TMA, and Et-CP₂Mg(ethyl cyclopentadienyl magnesium) as the precursor of Mg which servesas a p-type impurity are introduced to form the p-type AlGaN layer 107on the active layer 106 up to 0.01 μm. Then, the supply of TMA isstopped to grow the p-type GaN guiding layer 108 up to 0.1 μm and TMA isintroduced again to grow the p-type AlGaN cladding layer 109 up to 0.5μm. Moreover, the p-type GaN contact layer 110 is grown on the layer 109up to 0.1 μm. Thereafter, the supply of TMG and Et-CP₂Mg is stopped andtemperature lowering is started. When the substrate temperature reaches400 ° C., the supply of NH₃ is also stopped. When the substratetemperature reaches room temperature, the sapphire substrate 101 istaken out of the reactor.

[0065] The obtained wafer is set into a heat treatment furnace to applyheat treatment for the p-type conversion.

[0066] In this way, the laser wafer shown in FIG. 5 is prepared.

[0067] <Formation of Ridge Waveguide>

[0068] A ridge waveguide is formed as the index-guided type structure onthe prepared laser wafer through the following steps:

[0069] As shown in FIG. 6, a mask 115 having a plurality of slitsparallel to each other is formed on the surface of the p-type GaNcontact layer 110, and the exposed area of the nitride semiconductorlayer is partially etched by reactive ion etching (RIE).

[0070] In this case, as shown in FIG. 7, the etching is performed downto a depth where the p-type AlGaN cladding layer 109 is slightly left toform a recessed portion 201. Then, the mask 115 is removed to formnarrow ridge structures 118 of 5 μm-wide extending parallel to eachother. FIG. 7 shows two narrow ridge structures 118.

[0071] An SiO₂ protective film 111 is deposited on the wafer bysputtering as shown in FIG. 8.

[0072] Thereafter, a plurality of 3 μm-wide window portions 113 a forn-type electrodes are formed at the tops of the ridge structures 118 inthe SiO₂ protective film 111 by a standard photolithographic technique.

[0073] Nickel (Ni) with a thickness of 50 nm and subsequently Gold (Au)with a thickness of 200 nm are evaporated onto the SiO₂ protective film111 and the portion where the p-type GaN contact layer 110 is exposed toform the p-side electrode 113. Thus, the device structures each shown inFIG. 10 are formed on the device wafer.

[0074] <Bonding of a Cleavable Substrate to the Wafer>

[0075] Subsequently, as shown in FIG. 11, a GaAs single-crystalsubstrate 200 is bonded onto the p-side electrode 113 at the ridgewaveguide side of the wafer as to be connected electrically to the laserstructure. In this bonding step, the GaAs substrate 200 is aligned tothe GaN laser structure in such a manner that the crystallographicorientation of the GaAs crystal substrate is set to be parallel to thatof the GaN crystal layers, so that the cleavage of the GaAs crystal willcoincide with the GaN cleavage plane in the next cleaving step whereinthe desired laser resonator plane is formed by the cleavage of GaNcrystal. A GaAs single-crystal substrate of p-type conductivity is usedin this case. A Ti—Au thin film and Au—Sn thin film are previouslyformed in order by evaporation onto the surface of GaAs single-crystalsubstrate to be contacted to the p-side electrode 113 of the GaN crystallayer. The GaAs surface with the metal films and the electrode 113 arebrought into contact, and then pressurized to achieve the bonding ofboth the substrates.

[0076] <Irradiation of Light from Sapphire Side to the Wafer>

[0077] Subsequently, as shown in FIG. 12, the bonded wafer is irradiatedfrom the backside of and through the sapphire substrate 101 to theground layer 103 with a focused ultraviolet ray generated by ashort-wavelength high output laser such as a frequency quadrupled YAGlaser (with 266 nm wavelength), a KrF excimer laser (with 248 nmwavelength) or the like. The UV light beam may be applied uniformly ontothe entire interface between the sapphire substrate 101 and the groundlayer 103.

[0078] Whereas the sapphire substrate is almost transparent at 248 nmwhich is the wavelength of the laser beam used for the above UVirradiation, GaN of the ground layer absorbs the irradiation beam with asmall penetration depth because it has an absorption edge of 365 nm.Moreover, because of a large lattice mismatch (15%) present between thesapphire substrate and the GaN layer, extremely dense defects arepresent in the GaN crystal nearby the interface and thereby, absorbedlight is almost converted to heat. The temperature of an area of GaNnearby the sapphire substrate rapidly rises and thus, GaN is decomposedinto gallium and nitrogen. Therefore, a decomposed-matter area 150 ofthe nitride semiconductor is produced at the interface region in theground layer 103.

[0079] The decomposed-matter area 150 is provided for the purpose ofpromoting the crystal separation of the sapphire substrate 101 from theground layer 103 of GaN and AlN. The sapphire substrate is used only forthe manufacture of device. The wavelength of applied laser beam isselected from wavelengths absorbed by a GaN crystal layer and passingthrough the sapphire substrate. Therefore, as for the irradiatedinterface area in the GaN ground layer 103, direct crystal bonds betweensapphire 101 and GaN 103 are disconnected. Thus the GaN ground layer 103may be readily separated from the sapphire substrate 101 along thedecomposed-matter area 150.

[0080] <Separation of Sapphire and Laser Wafers>

[0081] After that, the sapphire substrate 101 is slightly heated toseparate the ground layer 103 with other crystal layers thereon from thesapphire substrate 101.

[0082] By this heating step, as shown in FIG. 13, the sapphire substrate101 is removed from the lamination i.e., the laser wafer of the bondedlaser body 100 and the support substrate 200, because atomic bondsbetween gallium and nitrogen are lost within the decomposed-matter area150.

[0083] After the removal of the sapphire substrate 101, the exposedsurface of the ground layer 103 is cleaned by dipping the laser waferinto a dilute hydrochloric acid solution or the like to remove residualmetallic Ga therefrom.

[0084] Ti with a thickness of 50 nm and Au with a thickness 200 nm aresuccessively evaporated onto the exposed surface of the laser wafer toform an n-side electrode 102.

[0085] The GaAs support substrate 200 may be thinned by lapping tofacilitate to cleave the laser wafer. Ti/Au electrode is evaporated ontothe surface of GaAs of the laser wafer.

[0086] <Cleaving of the Ground Layer>

[0087] As shown in FIG. 14, in the case of the laser wafer of the bondedlaser body 100 and the support substrate 200, the support substrate 200is cleaved together with the ground layer 103 along the linesperpendicular to the ridge waveguide extending direction at an intervalP which corresponds to the length of the final device.

[0088] At this cleaving step, the scribing (so-called notchingoperation) may be performed on the surface of the support substrate 200by using a diamond point. As a result, a plurality of laser bars 300 areobtained.

[0089] <Reflection Coating on Laser Bar Side Surface>

[0090] As shown in FIG. 15, dielectric multilayer reflection coatings302 are formed on both the fracture planes (cleavage planes) 301 of eachlaser bar 300 by a sputtering system or the like.

[0091] <Formation of Laser-Chips From Laser Bar>

[0092] As shown in FIG. 16, individual laser chips are obtained byfurther splitting the laser bar by means of second cleavage along thedirection parallel to the ridge waveguide extending direction.

[0093] <Assembling of Laser-Chip>

[0094] Each laser-chip of the bonded laser body 100 and supportsubstrate 200 is bonded via a Ti—Au thin film onto a chip carrier 10serving as a heat sink in such a manner that the ground layer 103 of thelaser body 100 is electrically connected to the chip carrier.

[0095] As described above, the laser structure formed on the A-face of asapphire substrate is disclosed. In addition, the laser structure ofridge waveguide type may be formed on the C-face of the sapphiresubstrate.

[0096] <Secondary Embodiment>

[0097] The second embodiment to be fabricated is a group III nitridesemiconductor laser device of a gain-guided type.

[0098]FIGS. 17 and 18 shows the gain-guided type group IIInitride-semiconductor laser device of the second embodiment. The membersof the device in those figures are the same as ones of the firstembodiment shown in FIGS. 3 and 4.

[0099] This device of the second embodiment is constructed with a laserbody 100, a support substrate 200 bonded onto the laser body 100, and anelectrically conductive chip carrier 10 serving as a heat sink bondedonto the support substrate 200. The laser body 100 comprises crystallayers 104 to 110 each made of a group III nitride semiconductor(Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1, 0≦y≦1) successively grown in orderon the ground layer 103. An electrode layer 113 a is formed on then-type GaN ground layer 103 through which the device is electricallyconnected to an external electrode. Whereas, the p-type GaN contactlayer 110 of the device is electrically connected through a stripewindow 213 a to the support substrate 200 and the chip carrier 10. Awindow film 213 made of GaAs oxide is formed between the laser body 100and the support substrate 200. The GaAs oxide window film 213 isprovided with the stripe window 213 a of GaAs extending along adirection normal to the cleavage plane of the nitride semiconductorlayers. The GaAs stripe 213 a serves as an electric current passagebetween the laser body 100 and the support substrate 200. It ispreferable that a joining metal film is provided between the GaAs oxidewindow film 213 and the p-type GaN contact layer 110 to bond the body100 and the support substrate 200.

[0100] As shown in FIG. 18, the body 100 of the laser device includes aplurality of crystal layers from the n-type GaN layer 103 and the p-typeGaN contact layer 110 which are layered as the same order as shown inFIG. 4. In this laser device, only the GaAs stripe 213 a provides theelectric current to the active semiconductor layer instead of the ridgewaveguide formed in the cladding layer so that the device is thegain-guided type. The GaAs oxide film makes insulation between the laserbody 100 and the support substrate 200 expect the GaAs stripe 213 a.

[0101] The device structure shown in FIGS. 17 and 18 is fabricated in asimilar manner as the first embodiment in which layered structure forthe device is formed through the metal-organic chemical vapor deposition(MOCVD) on an A-face sapphire substrate.

[0102] First, the laser wafer with the GaN semiconductor laser structureshown in FIG. 5 is prepared on the basis of the sapphire substrate.

[0103] Whereas, a GaAs oxide window film 213 with a plurality of GaAsstripes 213 a is formed on a GaAs single-crystal substrate 200 i.e.,support substrate as shown in FIG. 19. The interval of GaAs stripes 213a is approximately 200 μm and the width of each stripe is approximately2 to 5 μm for example. Those film and stripes may be formed in thefollowing steps 1) to 6):

[0104] 1) coating the surface of GaAs single-crystal substrate 200 witha photoresist;

[0105] 2) irradiate pertinent light through a mask having stripe windowsonto the photoresist layer;

[0106] 3) developing the photoresist layer on the substrate;

[0107] 4) settling the remaining photoresist down to the substrate toform given striped photoresist masks;

[0108] 5) oxidizing the exposed surface of the GaAs substrate other thanthe stripe photoresist masks to form a GaAs oxide window film 213 with aplurality of GaAs stripes 213 a. There are defined plural GaAs stripes213 a beneath the stripe photoresist masks; and

[0109] 6) removing the stripe photoresist masks from the substrate.

[0110] In this way, the GaAs oxide window film 213 with a plurality ofGaAs stripes 213 a is formed on the GaAs support substrate.

[0111] Subsequently, as shown in FIG. 20, the GaAs support substrate ofGaAs single-crystal 200 is bonded onto the p-type GaN contact layer 110of the wafer via the GaAs oxide window film 213 with a plurality of GaAsstripes 213 a as to be connected electrically to the laser structure. Inthis bonding step, the GaAs substrate 200 is aligned to the GaN laserstructure in such a manner that the crystallographic orientation of theGaAs crystal substrate is parallel to or coincides with that of the GaNcrystal layers, so that there will be appearance of the GaAs cleavagesurface or fractured plane matched for the GaN one in the next cleavingstep wherein a given laser resonator consists of the GaN cleavagesurface of the crystal layer.

[0112] A thin metal film such as In, Ni or the like may be previouslyformed by evaporation on at least one of contacting surfaces of GaAsoxide film 213 on the substrate 200 and the p-type GaN contact layer 110of the wafer in order that both the substrates come contact with eachother via the thin metal film sandwiched between the p-type GaN contactlayer 110 and the GaAs oxide window film 213. In this case, as shown inFIG. 24, it is preferable that the joining thin metal films 222 a and222 b are provided on the GaAs single-crystal substrate 200 and thep-type GaN contact layer 110 respectively in which slits 223 with anabout 10 μm width are formed along the edges of the GaAs stripe 213 a inthe GaAs oxide film 213. That is, as shown in FIG. 25, the slits 223define a narrowed current path CP of metal from the GaAs substrate 200through the GaAs stripe 213 a to the p-type GaN contact layer 110, whenthe joining thin metal films 222 a and 222 b are fused in the next step.

[0113] Anyway, the bonding surface of the support substrate is madecontact with the surface of cladding layer opposite to the ground layer103 with respect to the active layer of the laser wafer while beingpressurized and heated, and then close adhesion of both the substratesis achieved.

[0114] Subsequently, as shown in FIG. 21, an ultraviolet ray isirradiated through the sapphire substrate 101 to the ground layer 103 byusing a short-wavelength high output laser device. Namely the UVradiation is performed from the backside of sapphire substrate whilebeing converged by a converging lens. Since GaN absorbs UV light, thetemperature of the area of GaN nearby the sapphire substrate suddenlyrises and thus, GaN is decomposed into gallium and nitrogen, so that thedecomposed-matter area 150 of the nitride semiconductor is producedalong the light trace.

[0115] After that, the sapphire substrate 101 carrying the GaN layers isslightly heated and then, as shown in FIG. 22, the sapphire substrate101 is removed from the lamination i.e., laser wafer of the bonded laserbody 100 and support substrate 200 at the boundary of thedecomposed-matter area 150 of the ground layer 103.

[0116] Then, an n-side electrode layer 103 a is formed on the exposedsurface of the GaN ground layer 103 of the laser body 100.

[0117] After that, the cleaving step, the reflection layer formationstep and the assembling step are preformed in turn and then thesemiconductor laser device as shown in FIGS. 17 and 18.

[0118] According to the present invention, it is possible to utilize thenatural cleavage plane of nitride semiconductor for fabricating theresonator of the device by removing the substrate for the crystalgrowth. An atomically flat mirror facet is easily obtained, therebyreducing the optical scattering loss. As a result, continuousoscillation of laser is achieved and the same time a long life of thelaser device is obtained in practical.

What is claimed is:
 1. A method for fabricating a nitride semiconductorlaser device having crystal layers each made of a group III nitridesemiconductor (Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1, 0≦y≦1) layered inorder on a ground layer (Al_(x′)Ga_(1−x′))_(1−y′)In_(y′)N (0≦x′≦1,0≦y′≦1), the method comprising the steps of: forming a plurality ofcrystal layers each made of group III nitride semiconductor on a groundlayer formed on a substrate, the crystal layers including an activelayer; applying a light beam from the substrate side toward theinterface between the substrate and the ground layer thereby forming thedecomposed-matter area of a nitride semiconductor; separating the groundlayer with the crystal layers thereon from the substrate along thedecomposed-matter area; and cleaving the ground layer thereby forming acleavage plane of the crystal layers.
 2. A method for fabricating anitride semiconductor laser device according to claim 1, wherein thesubstrate is made of sapphire.
 3. A method for fabricating a nitridesemiconductor laser device according to claim 1, wherein the wavelengthof the light beam is selected from wavelengths passing through thesubstrate and absorbed by the ground layer in the vicinity of theinterface.
 4. A method for fabricating a nitride semiconductor laserdevice according to claim 1 further comprising, between said step offorming the crystal layers and said step of applying the light beamtoward the interface, a step of bonding a cleavable second substrateonto a surface of the crystal layers in such a manner that a cleavageplane of the second substrate substantially coincides with a cleavageplane of the crystal layers of the nitride semiconductor.
 5. A methodfor fabricating a nitride semiconductor laser device according to claim1, wherein the cleavable second substrate is made of a semiconductorsingle-crystal material.
 6. A method for fabricating a nitridesemiconductor laser device according to claim 5, wherein thesemiconductor single-crystal material is selected from a groupconsisting of GaAs, InP and Si.
 7. A method for fabricating a nitridesemiconductor laser device according to claim 1, wherein, in the step ofapplying the light beam toward the interface, the light beam is applieduniformly or entirely onto the interface between the substrate and theground layer.
 8. A method for fabricating a nitride semiconductor laserdevice according to claim 1, wherein, in the step of applying the lightbeam toward the interface, the interface between the substrate and theground layer is scanned with a spot or line of the light beam.
 9. Amethod for fabricating a nitride semiconductor laser device according toclaim 1 further comprising a step of forming a waveguide extending alonga direction normal to the cleavage plane of the nitride semiconductor.10. A method for fabricating a nitride semiconductor laser deviceaccording to claim 1, wherein the crystal layers of the nitridsemiconductor are formed by metal-organic chemical vapor d position. 11.A method for fabricating a nitride semiconductor laser device accordingto claim 1, wherein, in the step of applying the light beam toward theinterface, the light beam Is an ultraviolet ray generated from afrequency quadrupled YAG laser.
 12. A nitride semiconductor laser devicehaving successively grown crystal layers each made of a group IIInitride semiconductor (Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1, 0≦y≦1)comprising: a ground layer made of group III nitride semiconductor(Al_(x′)Ga_(1−x′))_(1−y′)In_(y′)N (0≦x′≦1, 0≦y′≦1); a plurality ofcrystal layers each made of group III nitride semiconductor formed onthe ground layer; a cleavable substrate bonded onto a surface of thecrystal layers opposite to the ground layer.
 13. A nitride semiconductorlaser device according to claim 12, wherein the device further comprisesa heat sink bonded onto the ground layer.
 14. A nitride semiconductorlaser device according to claim 12, wherein the device further comprisesa heat sink bonded onto the cleavable substrate.
 15. A nitridesemiconductor laser device according to claim 12, wherein the cleavablesubstrate has a cleavage plane coinciding with a cleavage plane of thecrystal layers of the nitride semiconductor.
 16. A nitride semiconductorlaser device according to claim 12, wherein the device further comprisesa waveguide extending along a direction normal to the cleavage plane ofth nitride semiconductor.
 17. A nitride semiconductor laser deviceaccording to claim 12, wherein the cleavable substrate is made of asemiconductor single-crystal material.
 18. A nitride semiconductor laserdevice according to claim 17, wherein the semiconductor single-crystalmaterial is selected from a group consisting of GaAs, InP and Si.
 19. Anitride semiconductor laser device according to claim 12, wherein thecleavable substrate is made of an electrically conductive material.